Study of the $$B_c^+ \rightarrow J/\psi D_s^+$$ B c + → J / ψ D s + and $$B_c^+ \rightarrow J/\psi D_s^{*+}$$ B c + → J / ψ D s ∗ + decays with the ATLAS detector

→ J/ψ D * + s are studied with the ATLAS detector at the LHC using a dataset corresponding to integrated luminosities of 4.9 and 20.6 fb -1 of pp collisions collected at centre-ofmass energies √ s = 7 TeV and 8 TeV, respectively.Signal candidates are identified through J/ψ → μ + μ -andWith a two-dimensional likelihood fit involving the B + c reconstructed invariant mass and an angle between the μ + and D + s candidate momenta in the muon pair rest frame, the yields of B + c → J/ψ D + s and B + c → J/ψ D * + s , and the transverse polarisation fraction in B + c → J/ψ D * + s decay are measured.The transverse polarisation fraction is determined to be ±± (B + c → J/ψ D * + s )/ (B + c → J/ψ D * + s ) = 0.38 ± 0.23 ± 0.07, and the derived ratio of the branching fractions of the two modes is B B + c →J/ψ D * + s /B B + c →J/ψ D + s = 2.8 +1.2 -0.8 ± 0.3, where the first error is statistical and the second is systematic.Finally, a sample of B + c → J/ψπ + decays is used to derive the ratios of branching fractions B B + c →J/ψ D + s /B B + c →J/ψπ + = 3.8 ± 1.1 ± 0.4 ± 0.2 and B B + c →J/ψ D * + s /B B + c →J/ψπ + = 10.4 ± 3.1 ± 1.5 ± 0.6, where the third error corresponds to the uncertainty of the branching fraction of D + s → φ(K + K -)π + decay.The available theoretical predictions are generally consistent with the measurement.( * )+ s decays: a colour-favoured spectator, b colour-suppressed spectator, and c annihilation topology, is also measured.From a naive prediction by spin counting, one would expect this fraction to be 2/3, while calculations [8,9] predict values of 0.41-0.48.This analysis is based on a combined sample of pp collision data collected by the ATLAS experiment at the LHC at centre-of-mass energies √ s = 7 TeV and 8 TeV corresponding to integrated luminosities of 4.9 and 20.6 fb -1 , respectively. The ATLAS detector, trigger selection and MonteCarlo samples ATLAS is a general-purpose detector consisting of several subsystems including the inner detector (ID), calorimeters and the muon spectrometer (MS).Muon reconstruction makes use of both the ID and the MS.The ID comprises three types of detectors: a silicon pixel detector, a silicon microstrip semiconductor tracker (SCT) and a transition radiation tracker.The ID provides a pseudorapidity 2 coverage up to |η| = 2.5.Muons pass through the calorimeters and reach the MS if their transverse momentum, p T , is above approximately 3 GeV.3 Muon candidates are formed either from a stand-alone MS track matched to an ID track or, in case the MS stand-alone track is not reconstructed, from an ID track extrapolated to the MS and matched to track segments in the MS.Candidates of the latter type are referred to as segmenttagged muons while the former are called combined muons.Muon track parameters are taken from the ID measurement alone in this analysis, since the precision of the measured 2 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and the z-axis along the beam pipe.The x-axis points from the IP to the centre of the LHC ring, and the y-axis points upward.Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the beam pipe.The pseudorapidity is defined in terms of the polar angle θ as η =ln tan(θ/2).

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